Oh, it can definitely fit and that is how many people go riding the first year. But it's against the law in my state -- or at least, it is against the law to do it safely (go figure). It is not safe for a baby under a year old to wear a helmet, because they don't have the neck strength, and it is very unsafe to wear a helmet in a carseat. (The seat isn't meant for a helmet so it pushes the neck forward.) California law doesn't prohibit babies under one year in bike trailers, but does require bike helmets for any kid riding in a trailer. So we'd either have to break the law or be unsafe.

My personal feeling: a baby in a carseat secured into a trailer, without a helmet, is just as safe as an older kid wearing a helmet in a trailer. And I am not really above scofflawing if there is a good reason. But she's not ready to ride in the trailer for reasons unrelated to safety, anyway. (She doesn't even like to face forward in a stroller -- she is still young enough that she really needs to be able to see Mom or Dad when the world is too overwhelming.)